1. Field of the Invention
The present invention relates to a transfer device that transfers a toner image on a surface of an image carrier onto a recording material nipped in a transfer nip formed by abutment between the image carrier and a nip forming member, and an image forming apparatus using the transfer device.
2. Description of the Related Art
Known is an image forming apparatus as described in Japanese Patent Application Laid-open No. 2006-267486 as the image forming apparatus of this type. The image forming apparatus forms a toner image on a surface of a drum-like photosensitive element by a well-known electrophotographic process. An endless intermediate transfer belt as an image carrier is made to abut against the photosensitive element so as to form a primary transfer nip. In the primary transfer nip, the toner image on the photosensitive element is primarily transferred onto the intermediate transfer belt. A secondary transfer roller as a nip forming member is made to abut against the intermediate transfer belt so as to form a secondary transfer nip. Furthermore, a secondary transfer opposing roller is arranged in a loop of the intermediate transfer belt. The intermediate transfer belt is nipped between the secondary transfer opposing roller and the above-mentioned secondary transfer roller. An earth is connected to the secondary transfer opposing roller at the inner side of the loop and a secondary transfer voltage is applied to the secondary transfer roller at the outer side of the loop. With this, a secondary transfer electrical field for moving the toner image electrostatically to the secondary transfer roller from the secondary transfer opposing roller is formed between the secondary transfer opposing roller and the secondary transfer roller. Then, the toner image on the intermediate transfer belt is secondarily transferred onto a recording sheet fed into the secondary transfer nip at a timing of being synchronized with the toner image on the intermediate transfer belt with actions of the secondary transfer electric field and a nip pressure.
With this configuration, if a recording sheet with large surface irregularities, such as Japanese paper, is used as the recording sheet, a shading pattern in accordance with the surface irregularities is easily generated on an image. The shading pattern is generated when a sufficient amount of toner is not transferred onto recesses on the sheet surface and image density on the recesses is lower than that on protrusions.
Then, in the image forming apparatus described in Japanese Patent Application Laid-open No. 2006-267486, as the secondary transfer voltage, not a voltage composed of a DC component only but an AC voltage in which an AC component is superimposed on the DC component is applied. According to Japanese Patent Application Laid-open No. 2006-267486, although specific reasons have not been disclosed, by using the secondary transfer voltage, toner reciprocates between the surface recesses on the recording material and the image carrier, so that the toner can make contact with the surface recesses on the recording material. This makes it possible to suppress transfer failure of the toner onto the surface recesses on the recording material. In addition, in Japanese Patent Application Laid-open No. 2006-267486 discloses an experimental result indicating that if such a secondary transfer voltage is applied, generation of the shading pattern can be suppressed in comparison with a case where the secondary transfer voltage composed of the DC component only is applied.
However, the applicants have found by experiments that a sufficient image density cannot be obtained on the recesses on the surface of a recording sheet in some cases with the configuration disclosed in Japanese Patent Application Laid-open No. 2006-267486. The applicants have explored the reasons therefor and have found the following fact, which will be described in detail with reference to some drawings.
FIG. 1 is an enlarged configuration view illustrating an example of a secondary transfer nip.
In FIG. 1, an intermediate transfer belt 531 is pressurized toward a secondary transfer roller 536 with a secondary transfer opposing roller 533. The secondary transfer opposing roller 533 abuts against a rear surface of the intermediate transfer belt 531. With the pressurization, a secondary transfer nip on which a surface of the intermediate transfer belt 531 and the secondary transfer roller 536 abut against each other is formed. A toner image on the intermediate transfer belt 531 is secondarily transferred onto a recording sheet P fed to the secondary transfer nip. A secondary transfer voltage for secondarily transferring the toner image is applied to any one of the secondary transfer opposing roller 533 and the secondary transfer roller 536 and the other of them is grounded. While the toner image can be transferred onto the recording sheet P when the transfer voltage is applied to any of the rollers, a case where the secondary transfer voltage is applied to the secondary transfer opposing roller 533 and toner having negative polarity is used is described as an example. In this case, in order to move the toner in the secondary transfer nip to the secondary transfer roller 536 from the secondary transfer opposing roller 533, a voltage of which time average value is at the negative polarity, which is the same as the polarity of the toner, is applied as the secondary transfer voltage as an alternating voltage.
FIG. 2 is a waveform chart illustrating an example of the waveform of the secondary transfer voltage to be applied to the secondary transfer opposing roller 533.
The waveform of the secondary transfer voltage is a sine wave as illustrated in FIG. 2 and the reference symbol “Vave” in FIG. 2 indicates a time average value of the secondary transfer voltage. The reference symbol “Vt” in FIG. 2 indicates a peak value of a voltage (hereinafter, referred to as “supply voltage”) having polarity (negative polarity) in the transfer direction in which the toner is transferred onto the recording sheet P from the intermediate transfer belt 531 in the secondary transfer nip. The reference symbol “Vr” in FIG. 2 indicates a peak value of a voltage (hereinafter, referred to as “return voltage”) having polarity (positive polarity) in the direction in which the toner is returned to the intermediate transfer belt 531 from the recording sheet P in the secondary transfer nip.
When an AC voltage having the AC component only without the DC component is used as the secondary transfer voltage, the toner can be made to reciprocate between the intermediate transfer belt 531 and the recording sheet in the secondary transfer nip. However, with the AC voltage having no DC component, the toner is made to reciprocate simply and cannot be transferred onto the recording sheet P. Therefore, a voltage in which the AC voltage is superimposed on the DC component is required to be used as the secondary transfer voltage and the time average value Vave of the secondary transfer voltage is required to be set to be at the polarity (negative polarity) in the transfer direction in which the toner is transferred onto the recording sheet P from the intermediate transfer belt 531. With this, the toner can be made into a state of being transferred onto the recording sheet P after having passed through the secondary transfer nip while reciprocating between the intermediate transfer belt 531 and the recording sheet.
The applicants have observed the reciprocating movement of the toner with experimental devices and have found the following fact.
If the secondary transfer voltage is started to be applied, first, only an extremely small amount of toner particles present on a surface of a toner layer on the intermediate transfer belt 531 escape from the toner layer so as to move toward recesses on a surface of a recording sheet with an action of an electric field when a supply voltage is applied. At this time, almost all of the toner particles in the toner layer still remain in the toner layer. The extremely small amount of toner particles having escaped from the toner layer enter the recesses on the surface of the recording sheet, and then, return back to the toner layer from the recesses with an action of an electric field when a return voltage is applied. At this time, the returning toner particles collide with the toner particles remaining in the toner layer, so that the adhesive force of the toner particles in the toner layer is weakened. Then, when the supply voltage is applied next, more toner particles than those in the first time escape from the toner layer so as to move toward the recesses on the surface of the recording sheet. If such a series of behavior is repeated, the number of toner particles that escape from the toner layer and enter the recesses on the surface of the recording sheet P increases gradually. As a result, a sufficient amount of toner particles are transferred into the recesses on the surface of the recording sheet P and generation of a shading pattern in accordance with the surface irregularities of the recording sheet P on an image can be suppressed.
Furthermore, the applicants have found that transfer performance onto the recesses have a high correlation with the peak value Vr of the return voltage from results of experiments performed by using various types of recording materials. That is to say, while one tries to obtain a sufficient image density even on the recesses, unless the peak value Vr of the return voltage is large to some extent, the sufficient transfer performance onto the recesses is not obtained and the image density on the recesses is insufficient even if other devises including an increase in the application time of the peak value Vr of the return voltage are made. The reason for this is as follows.
In order to obtain high transfer performance onto the recesses, it is insufficient that the toner moved to the recording material is returned back to the image carrier with the return voltage only, and the returned toner is required to be made to collide with the toner layer on the image carrier so as to weaken the adhesive force of the toner in the toner layer. Otherwise, the same toner particles reciprocate only and the number of toner particles that escape from the toner layer and enter the recesses on the surface of the recording material cannot gradually increase. That is to say, the key to obtain high transfer performance onto the recesses is to generate collision that is strong enough to weaken the adhesive force of the toner in the toner layer on the image carrier with the toner returned back from the recording material. Furthermore, the strength of the collision depends on the peak value Vr of the return voltage. The above-described collision cannot be generated unless the peak value Vr of the return voltage is large to some extent.
The above-mentioned reason was first found by the applicants through observation of the above-mentioned reciprocating movement with experimental devices.
The applicant has developed a technique in which a peak-to-peak voltage of an AC component of a secondary transfer voltage is set to a value that is larger than four times the absolute value of a DC component in Japanese Patent Application No. 2010-183301 (hereinafter, referred to as “previous application”). If the secondary transfer voltage is applied, the peak value Vr of the return voltage becomes large sufficiently. Therefore, sufficient transfer performance onto the recesses on the recording material is obtained, so that image density of the recesses can be enhanced sufficiently.
However, as a result of further studies by the applicants, with the configuration disclosed in the above-mentioned previous application, a plurality of white spots are generated on an image generated on the recording material in some cases. The applicants have explored the reason why the white spots are generated and have found the following fact.
In order to form an image having high image quality on a recording material with irregularities on a surface thereof, sufficient transfer performance is required to be obtained on both the recesses and the protrusions on the surface. The transfer performance onto the protrusions depends on a time average value Vave of a secondary transfer voltage when the secondary transfer voltage in which an AC component is superimposed on a DC component is applied. That is to say, high transfer performance onto the protrusions cannot be obtained and a sufficient image density cannot be obtained on the protrusions unless the absolute value of the time average value Vave of the secondary transfer voltage is increased and is set to sufficiently large to polarity in which the toner is transferred onto the recording material from the image carrier.
With the configuration disclosed in the previous application, in order to obtain high transfer performance onto the recesses, the peak-to-peak voltage of the AC component of the secondary transfer voltage is set to be a value that is more than four times the absolute value of the DC component. The waveform of the secondary transfer voltage used in the configuration is a sine wave. Therefore, the DC component of the secondary transfer voltage is identical to the time average value Vave of the secondary transfer voltage. Accordingly, in the configuration disclosed in the previous application, if the absolute value of the time average value Vave of the secondary transfer voltage (absolute value of the DC component) is set to be large in order to obtain high transfer performance onto the protrusions, the peak-to-peak voltage of the AC component also increases in accordance therewith.
The peak-to-peak voltage of the AC component is identical to a differential value between the peak value Vt of the supply voltage and the peak value Vr of the return voltage. Therefore, if the peak-to-peak voltage increases, the peak value Vr of the return voltage becomes a sufficiently large value and high transfer performance onto the recesses can be obtained. However, as the peak-to-peak voltage increases, the peak value Vt of the supply voltage also increases. In particular, when the secondary transfer voltage is a voltage in which the AC component having a sine wave is superimposed on the DC component as in the configuration disclosed in the previous application, the peak value Vt of the supply voltage is the sum of a value that is half the peak-to-peak voltage of the AC component and the absolute value of the DC component. Therefore, if the absolute value of the DC component increases and the peak-to-peak voltage of the AC component also increases in accordance therewith so as to obtain a sufficiently large peak value Vr of the return voltage, the absolute value of the peak value Vt of the supply voltage will indicate an extremely large value.
If the absolute value of the peak value Vt of the supply voltage indicates a large value, electric discharge is generated in the transfer nip during an application period of the supply voltage. When the electric discharge is generated, the toner that has received the electric discharge is charged to have polarity opposite to normal charged polarity on the electric discharge generation places, for example. For these reasons, the toner does not adhere onto the recording material. Therefore, white spots appear on portions of the image that correspond to the electric discharge generation places. In this sense, the configuration disclosed in the previous application can make it difficult to obtain sufficient image densities on both the protrusions and the recesses on the recording material.
In view of the foregoing, there is a need to provide a transfer device and an image forming apparatus that can obtain sufficient image densities on both the recesses and the protrusions on a surface of a recording material with large surface irregularities without generating white spots (white out) in an image when the image is formed on the recording material.